In designing a wavelength division multiplexed optical transmission link including multiple spans of fiber with optical amplifiers interposed between the spans, conventional design approaches have often assumed that the noise figure for the system is spectrally flat and equal in magnitude to the worst case noise figure for the system. Designers of these systems calculate a desired signal to noise ratio (SNR) as a function of the number of spans in the system, and select a launch power for wavelength signals input to the system that ensures that all channels will achieve the desired SNR, even at wavelengths having the highest noise figure. Generally, designers apply the same launch power to all wavelength signals.
Some systems implementing Erbium doped amplifiers have been designed to pre-emphasize signal input powers to address the predictable wavelength dependent gain characteristics inherent to the Erbium doped amplifiers. In typical implementations, absent the use of other gain flattening elements, this often results in a primarily positive or a relatively flat slope of the input signal spectrum as wavelengths increase. In addition, these systems often require that the total input signal power always remain at a constant level.